1. Field of the Invention
This invention relates to a surface-hardened chain used as a load chain of an electric chain block or a pneumatic chain hoist, a chain of a chain conveyor or the like.
2. Description of Related Art
In this type of the chain, it is required to have higher wear resistance and fatigue resistance because a very large loading is applied to the chain and the frequency in use is high. And also, it is required to have higher strength and toughness because an impact load is applied to the chain. In the conventional chain, therefore, there has been used a surface-hardened chain link formed by subjecting a chain link to gas carburizing, quenching and tempering treatments.
As sectionally shown in FIG. 3, such a surface-hardened chain link is composed of an outermost surface layer 10, a hardened layer 11 enclosed with the outermost surface layer 10 and having a high carbon tempered martensite structure, and a core portion 12 enclosed with the hardened layer 11 and having a low carbon tempered martensite structure.
In the conventional surface-hardened chain, Mn-B steel (SAE15B24), Ni-Cr-Mo steel (JIS SNCM220, SAM8620), Ni-Mo steel (SAE4620), Ni-Cr-Nn-Mo-B steel (see JP-A-61-276956) and the like were generally used as a starting material.
However, the surface-hardened chains made frrm these starting materials were insufficient in the wear resistance, fatigue resistance, strength and toughness as mentioned below.
In the surface-hardened chain made from Mn-B steel, Ni-Cr-Mo steel, Ni-Cr-Mn-Mo-B steel or the like, oxidation at crystal grain boundary (intergranular oxidation) was caused in a surface layer of a chain link by gas carburizing, so that the wear resistance and fatigue resistance in the surface layer were considerably deteriorated to bring about the premature degradation of the surface layer and also the strength and toughness were poor.
In the surface-hardened chain made from Ni-Mo steel, retained austenite was existent in the surface layer and hence the wear resistance and fatigue resistance in the surface layer were considerably deteriorated to bring about the premature degradation of the surface layer and also the toughness in the surface layer was low.